<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>添加选项</title>
</head>
<body>
  <form>
    <select name="location" id="selLocation">
      <option value="Sunnyvale, CA">Sunnyvale</option>
      <option value="Los Angeles, CA">Los Angeles</option>
      <option value="Mountain View, CA">Mountain View</option>
      <option value="">China</option>
      <option>Australia</option>
    </select>
  </form>
  <script>
    const selectbox = document.forms[0].elements.location;
    const newOption = new Option('Option text', 'Option value');  // 只能在最后插入新选项，如果想放到不同地方，应该使用 insertBefore()

    selectbox.add(newOption, undefined);
  </script>
</body>
</html>